Depends on your local micro climate and how far north you are. Also depends if you are doing seeds or clones - seedlings can be put out earlier. Clones you usually don't want to put out until mid or late May or they will flower on you. Don't make the mistake I see a lot of people do and put them out in March or early April only to have them killed by late frosts and storms. I usually shoot to plant by mid-May in Mendo, any earlier than that is too risky for the weather.

In Central Cali, I know people who start indoors on Jan 1 and move outdoors as soon as threat of frost is low. Plants are usually protected with little humidity tents to protect from the cold. They also plant a crop in March so as to pull two harvests per season.